1. Field of the Invention
This invention relates to bisimides and more particularly relates to white N,N'-alkylene-bis-tetrabromophthalimides and to a process for preparing them.
2. Description of the Prior Art
As described in U.S. Pat. Nos. 3,624,024 (Caldwell et al.) and 3,873,567 (Cyba), British Pat. No. 1,287,934 (Raychem), and Sydney M. Spatz and Herman Stone, "Some N-Substituted Tetrabromophthalimide Fire-Retardant Additives," INDUSTRIAL AND ENGINEERING CHEMISTRY PRODUCT RESEARCH AND DEVELOPMENT, Volume 8, pp. 397-398 (1969), it is known that N,N'-alkylene-bis-tetrabromophthalimides can be prepared and that they are useful as flame retardants for many normally flammable materials. However, as indicated by Raychem's Example 16, and as can be determined by repeating Cyba's Example VII, prior art processes for preparing these bisimides have resulted in poor yields and colored products when diaminoalkanes other than hexamethylenediamine have been used as reactants. The poor yields have made these processes economically unattractive, and the coloration of the products has prevented their use as flame retardants in many applications.